speed drop over phone extension

until quite recently i used to regularly get connection speeds of 1.8 MBps(i know, difficult to believe isn't it) but recently this has dropped to roughly 450 Kbps. the line will support 3 MBps . i dragged the whole lot into the hall and connected straight into the master socket on the phone connection (front plate removed) and i got approx 3MBps. i thought i'd cracked it so bought a new 10 metre long phone extension but only got 450KBps again.i'm going wireless with the BT hub but wonder if anyone has an explanation for this.

cheers